Individual VNIR and SWIR lenses are often used together in airborne imaging sensors designed for high resolution wide wavelength range imaging applications such as reconnaissance and surveillance or mapping. For these applications, each lens must meet strict optical requirements over their individual bands: the VNIR lens covering a wavelength range from 450 nm to 900 nm and the SWIR lens covering a wavelength range from 900 nm to 2450 nm. Each lens typically utilizes a number of glass elements to provide the necessary optical performance over its individual band. Such wide wavelength range lenses must also provide high transmission, high resolution, low distortion, and a flat image over large field of view to be useful.
U.S. Pat. No. 6,208,459 (Mar. 27, 2001) discloses a wide band lens made out of MgO and CaF2 materials. However, this lens has drawbacks. It has a relatively small field of view (FOV), and is not very compact. In addition, both materials are expensive. CaF2 is also a very soft material, making fabrication of lens elements difficult, especially aspheric elements.
The design of SWIR lenses has been described in “The Design of SWIR Imaging Lenses Using Plastic Optics” by R. Hamilton Shepard, Proc. of SPIE Vol. 8489 84890A-1. These lenses utilize the mix of elements made out of plastic materials and glass and are corrected for the spectrum 900 nm-1700 nm. Absorption in the plastics, especially for the wavelength above lμm, is very high and significantly reduces the lens transmission. Also, large field of view lenses cannot be developed because of plastics' low indices of refraction.
U.S. Pat. No. 8,289,633 (Oct. 16, 2012) discloses a lens corrected over a wide wavelength range of 315 nm-1100 nm. However, it cannot be used for the SWIR wavelength range above 1100 nm.
U.S. Pat. No. 8,817,392 (Aug. 26, 2014) discloses a wide field apochromatic lens. However, it is also corrected only for the VNIR spectrum from 550 nm-950 nm. Another example of a lens corrected for the VNIR spectrum is disclosed in U.S. Pat. No. 7,271,965 (Sep. 18, 2007). However, it too suffers from a limited wavelength range.
A wide wavelength range lens has to be apochromatic in order to obtain residual color (or secondary color) correction. That means that the lens optical groups and elements in these groups shall be configured and positioned relative to one another so that the focal length of the lens is the same at wavelengths not only at the ends of the spectrum of interest, but at intermediate wavelengths as well. Without such aberrational correction, the focal length of the system would vary undesirably with the wavelength of light received from the object, causing chromatic aberration or color fringes to appear at the edges of the projected image of the object.
Apochromatic correction has to be done for at least one intermediate wavelength to have good lens performance over a wide wavelength range. Correction at more than one intermediate wavelength enables good optical performance throughout the whole spectrum. Apochromatic correction requires glasses with special dispersive properties through the whole spectrum of interest. At the same time optical powers of lens groups and elements must be maintained to provide the necessary basic optical performance parameters.
Glasses of normal dispersion, which have an almost linear decrease in refractive index with increasing wavelength, are used to produce achromatic objectives. In this case, remaining secondary spectrum (also called lateral color) produces greenish or purple fringes on images of sharp edges. The higher quality apochromatic objectives use glasses having an abnormal partial dispersion; these glasses refractive index changes with wavelength more rapidly in either the blue or red region. As a result, apochromats have a high degree of chromatic correction in which three or more wavelengths can have the same image location.
The contribution of the optical element to the total axial color is proportional to the square of axial marginal ray height at the lens, its optical power, and it is reciprocal of Abbe number of lens.
The Abbe number V in terms of refractive indices for the VNIR/SWIR 450 nm-2450 nm spectrum is determined by:V=(n1450−1)/(n450−n2450)
Where n is the refractive index of glass for the specific wavelength of interest.
The dispersive characteristics of various glasses may be compared by plotting the relative partial dispersion Px,y versus the Abbe-number V. These quantities share a linear correspondence for most optical glasses and therefore plot along a single straight line. Glasses exhibiting this behavior are referred to as “normal dispersion glasses”. The partial dispersion P for the wave lengths range x-y of these glasses can be approximately described by the following equation:Px,y≈ax,y+bx,y·V 
where ax,y and bx,y are constants. Glasses which deviate significantly from the line described by this equation are called “abnormal dispersion glasses”. For any glass, the deviation of the partial dispersion from the “normal line” can be represented by the quantity ΔPx,y. A more general expression for Px,y is then given by the following equation:Px,y=ax,y+bx,y·V+ΔPx,y 
The partial dispersion for the wide VNIR/SWIR spectrum is determined by the following equation:P=(n1450−n2450)/(n450−n2450)
FIG. 1 depicts known Abbe numbers and relative partial dispersions 100 for Schott glasses.
The orthoscopic property (low distortion) of a lens is also important, as it provides a precisely scaled image of the target because the variation of focal length across the field is minimized. This feature is very important for airborne sensors intended for the identification and accurate mapping of objects.
Existing wide wavelength range lenses are not orthoscopic and their residual distortion is not less than 1%, so the precise measurements of the target characteristics and location cannot be performed without significant post processing of the image.
Therefore, there exists a need in the art for a high performance VNIR/SWIR airborne sensor lens with a wide wavelength range that is simultaneously low distortion or orthoscopic. Further, the application of a single high performance lens covering both VNIR and SWIR bands allows a sensor system with reduced size, weight, power, and complexity (SWAP-C).